"Our findings suggest that [eating] carotenoid-rich foods could help prevent or delay the onset of [amyotrophic lateral sclerosis (ALS)]," says researcher Kathryn C. Fitzgerald, a doctoral student in the department of nutrition at Harvard School of Public Health in Boston.

This research shows the carotenoids beta-carotene and lutein offered the most benefit. Beta-carotene can be found in carrots, squash, and sweet potatoes. Lutein is in dark green vegetables such as spinach and kale.

Lower ALS Risk

The study involved more than 1 million people, in which there were 1,093 cases of ALS.

The researchers recorded the diets of each participant during the preceding year, including detailed information on the use of multivitamins, vitamins C and E, and beta-carotene supplements.

People who ate foods with the most carotenoids had a 25% lower risk of ALS compared to people who ate the least.

Those who ate lots of carotenoids also tended to exercise more, have more advanced education, take in higher amounts of dietary vitamin C, and were generally more likely to take vitamin C and E supplements.

The effect was stronger among people who had never smoked; smoking can affect carotenoid levels.

Beta-Carotene and Lutein

Other carotenoids -- αlpha-carotene, beta-cryptoxanthin, and lycopene -- were not associated with a lower risk of ALS, nor were neta-carotene supplements.

Similarly, there was no link between vitamin C supplements, or dietary vitamin C intake, and the risk of ALS. There was also no significant evidence that vitamin E supplements helped.

Earlier research from Harvard showed that taking vitamin E supplements for more than five years or having a diet rich in vitamin E is associated with a lower risk of ALS. But this study suggests that vitamin E alone can't explain the link.

The study is one of the largest to look at dietary factors and risk of ALS, Fitzgerald says.

It's not clear from this study how often or how many brightly colored fruits and vegetables it's necessary to eat to lower the risk of ALS. "Right now there's no specific amount, or we can't say how much you should eat," says Fitzgerald.